The goal of this program is a physiological and anatomical analysis of the central neural mechanisms participating in circadian rhythm regulation. Circadian rhythms are an ubiquitous feature of organisms and in mammals, they appear to be genetically determined functions of endogenously generated oscillatory mechanisms. A number of these functions have profound effects on behavior and neuroendocrine regulation in the CNS. Previous studies have shown that the probable site of the "master clock" for circadian rhythm regulation in mammals resides in the suprachiasmatic nucleus in the hypothalamus. Visual input directly from the retina and indirectly from other visual centers are involved in synchronization of circadian rhythms. Very little is known as to the precise areas of the suprachiasmatic nucleus involved, the mechanisms involved, the interaction of secondary and primary synchronizers and oscillators involved and the anatomical connections necessary for the generation of circadian rhythms. The physiological studies proposed here include chronic multiple unit recordings in the suprachiasmatic nucleus and surrounding areas of the hypothalamus of the awake rat. Anatomical studies are on the afferent and efferent connections of the suprachiasmatic nucleus. These studies are designed in order to determine the precise nature and some basic mechanisms of circadian rhythm generation in the mammal. The anatomical studies will elucidate the differential connections of the areas invloved in primary (and secondary) oscillation.